Chromatography is one of the oldest chemical analysis methods in which a mixture is separated into individual chemical components. It thus becomes simpler to make a qualitative or quantitative determination of the chemical components in a mixture. In gas chromatography the mixture is guided through a separation column by means of an inert carrier gas: the mobile phase, usually helium or hydrogen. The separation is based on the differential interactions between the different chemical components in the mobile phase and an immobilized stationary phase: a liquid or solid material with which the inner wall of the separation column is covered or which is arranged on an inert carrier material in the separation column. The retention time of a chemical component in the separation column is a function of the measure of interaction with the stationary phase, the type and the quantity of stationary phase, the length and diameter of the separation column, the type of carrier gas, the flow speed and the temperature. The different chemical components will in principle now leave the separation column at different points in time. These points in time can be determined by guiding the outflow from the separation column to a detector. The different chemical components then appear as more or less sharp ‘peaks’ in the output of the detector: the chromatogram.
A TCD (Thermal Conductivity Detector) is usually used as detector in gas chromatography. Changes in the thermal conductivity of the outflow from the separation column can be detected herewith. This conductivity is compared to the conductivity of the pure carrier gas. Because most substances have a conductivity which is much lower than that of the carrier gas used, the conductivity is decreased when a component passes through, and this produces a difference signal. The TCD comprises a temperature-dependent electrical resistor placed in a detector body with a constant temperature. The outflow from the separation column is guided along the resistor. When there is a constant electric current through the resistor, there is normally speaking a stable heat flow from the resistor to the detector body. When a component passes through, the thermal conductivity of the gas enclosing the resistor drops. The resistor is then less well able to relinquish its heat and heats up. Because the electrical resistance of the resistor is temperature-dependent, it will therefore also change. This change is usually measured using a wheatstone bridge. Because practically all components, organic or inorganic, have a thermal resistance substantially different from the carrier gas, practically all components in the outflow can be detected by means of a TCD. The TCD is therefore a more or less universal detector.
In addition to a non-destructive TCD, other types of destructive detectors can also be utilized, such as an FID (Flame Ionization Detector), a PID (Photo Ionization Detector) an ECD (Electron Capture Detector) and an MDD (Micro Discharge Detector), these detectors being more sensitive to specific groups of components, see for instance WO 2007/081416 A1. The detectors in which ionization takes place are placed after, i.e. downstream of, the TCD and the ionization thus takes place downstream of the TCD.
In addition to being determined by the properties of the separation column, the injector and the carrier gas and the measure of control of sample injection, gas flows, pressures, temperatures and so on, the quality of a system for gas chromatography is particularly also determined by the properties of the detector or detectors. The sensitivity, accuracy and precision of the detector or detectors are preferably as high as possible. The invention now provides a solution for the purpose of influencing or improving the response of a TCD.